Polymer substrates with a large number of microfibers on a surface have a wide variety of potential applications. Such microstructured polymer films may be applied to a surface in order to decrease the gloss of the surface. Other surfaces which may benefit from the application of materials having increased surface area due to the presence of a large number of microfibers include carrier webs for use with adhesive tapes. Polymer surfaces covered with a plurality of microfibers also typically have a soft or cloth-like feel and can provide a low friction surface. Polymer sheet materials with smooth planar surfaces are often treated to provide fibers or fiberlike features protruding from at least one major surface. Alteration of a surface in this manner can produce a number of effects, e.g., a decorative appearance, the dispersion of incident light, increased wicking of fluids and/or a low friction surface.
A variety of methods for producing polymer films having a surface with a suede-like feel are known. For example, one of the oldest methods of achieving this effect is called flocking. This involves attaching one end of chopped fibers to a planar surface. Various methods have been used to position the fibers perpendicular to the planar surface (e.g., U.S. Pat. No. 3,973,059 or U.S. Pat. No. 5,403,884). Woven textiles are often passed through a napping machine which pulls loops of small strands from the woven article. The small pulled fibers may break or simply form a loop. The overall napping process typically imparts a soft feel to the napped surface of the article. Another approach which has been used to alter the surface of materials such as leather is to abrade the surface with abrasives such as sand paper. Processes of this type are used to make suede leather. A suede-like feel has been imparted to the surface of polymer foam materials by heat skiving the surface so that the thin sidewalls of the ruptured foam cells provide a soft feel to the treated surface (see, e.g., U.S. Pat. Nos. 3,814,644 and 3,607,493). Yet another method, such as disclosed in U.S. Pat. No. 5,403,478, involves bonding a non-woven sheet onto a plastic film. A suede-like feel has also been achieved by the extrusion of fibers onto a thermoplastic polymer film and heat bonding the fibers to the film (see, e.g., U.S. Pat. Nos. 3,152,002, 4,025,678 and 5,403,884).
Several patents (e.g. U.S. Pat. Nos. 5,116,563; 5,230,851; and 5,326,415) disclose a substrate having a plurality of tapered prongs on a surface. The prongs are formed by depositing islands of heated, thermally sensitive material (e.g., a thermoplastic material) onto the moving substrate surface such that a velocity differential exists between the depositing thermally sensitive material and the underlying substrate surface. The tapered prongs typically have a base diameter of about 700-1300 microns and heights of about 500-2000 microns. Other methods of forming tapered thermoplastic projections on an underlying sheet have also been reported. U.S. Pat. No. 3,027,595 discloses the formation of an artificial velvet fabric having a plurality of pile-like projections. The projections are formed by contacting a thermoplastic sheet with the heated surface of a drum having a multiplicity of closely spaced conical depressions in its surface. The exemplary pile-like projections disclosed have a base diameter of about 150 microns and a length of about 3000 microns (3 mm). U.S. Pat. No. 5,407,735 discloses a napped polyester fabric having sheath-core polyester fibers with a tapered tip. The fibers typically have a fineness in the range of 2 to 6 deniers and pile lengths of about 3 mm.
In order for the articles containing microstructured polymer materials to realize their full potential, versatile, inexpensive methods of fabricating such polymer materials must be available. Current methods typically only permit the generation of polymer substrates with limited types of microstructure configurations. A need, therefore, continues to exist for improved methods of producing polymer substrates having a surface with a napped texture. Such methods would preferably permit the production of polymer substrates with a defined microscopic pattern. Optimally, the method would also permit the introduction of macroscopic structural features (e.g., via embossing) and/or would allow the choice of generating a microscopic pattern on either all or a portion of the surface.
The application provides a polymer substrate having a plurality of microfibers projecting from at least one major surface. The microfibers are integral with and have the same composition as the underlying substrate, i.e., the microfibers and the underlying substrate form a unitary construction. The microfibers extend from the underlying major substrate and may have a variety of shapes. For example, the microfibers may have any of a number of cross-sectional shapes including squares, triangles, circles, ovals, rectangles or other geometric shapes as well as more irregular shapes. The placement of the microfibers on the surface may be random or in a predetermined array.
In one embodiment, a unitary polymer substrate which includes a plurality of frayed-end microfibers is provided. The microfibers themselves can include one or more surfaces having a plurality of microfibrils, i.e., microfibers of even smaller dimensions protruding from a surface of the larger microfibers. The microfibrils also typically have frayed ends. Unitary polymer films with a plurality of frayed-end microfibers typically have an extremely high surface area (e.g., as measured by nitrogen adsorption and/or electron microscopy).
A unitary polymer substrate having a napped surface which includes a plurality of microfibers having an expanded cross-section shape is also provided. The expanded cross-section shaped microfibers typically have an average maximum cross-sectional dimension of no more than about 200 microns and, preferably no more than about 100 microns. As used herein, xe2x80x9cexpanded-cross section shapexe2x80x9d is defined as a shape having a cross-sectional surface area which increases and then decreases along a perpendicular vector away from the surface of the unitary polymer substrate thereby creating a bulge in the microfiber. The cross-sectional surface area is measured in a plane parallel to the major surface of the polymer substrate from which the microfiber extends. The bulge may be the tip end (xe2x80x9cexpanded-head shapexe2x80x9d) and/or in the middle of the microfiber. Microfibers of this type may have more than one expanded cross-sectional portion (xe2x80x9cbulgexe2x80x9d) along their length, e.g., microfibers generated using an open cell foam as a template surface.
Another polymer substrate with a napped surface is described herein. The substrate is a unitary polymer substrate which includes a plurality of tapered microfibers projecting from the surface. Such tapered micofibers typically have an average maximum base cross sectional dimension of no more than about 200 microns and an average maximum half height cross sectional dimension of no more than about 100 microns. The average height of the tapered micofibers is typically at least about 400 microns and preferably about 500 to about 2,000 microns.
The present napped polymer surfaces may be prepared by a number of different methods. One method includes contacting a surface of a polymer substrate with an abrasive surface in a reciprocating manner to form a napped polymer surface including a plurality of frayed-end microfibers.
Polymer surfaces having a plurality of projecting expanded cross-section shaped microfibers may be produced by a method which includes laminating a polymer substrate to a resilient template surface having a plurality of microdepressions. During the lamination process softened material from the surface of the polymer substrate is forced into the microdepressions thereby forming a plurality of microprojections extending from the substrate surface. If the surface of the polymer substrate is maintained in a sufficiently softened while it is delaminated from the template surface, the microprojections can be stretched such that a plurality of microfibers extending from the polymer surface are generated prior to the debonding of the polymer substrate surface from the template surface. In other words, a blob of the softened polymer remains entrapped in the microdepression for a period of time while a stem of polymer connecting the blob to the underlying surface is drawn out. The stem increases in length while the polymer surface is cooling until the point where the blob of polymer is pulled out of the microdepression.
Another method of producing unitary polymer substrates having a plurality of microfibers includes laminating two thermoplastic polymer substrates (e.g., films) to opposite sides of a template film having a plurality of microscopic holes therethrough. The template film is typically either coated with or formed from a release material such as a silicone release material. The thermoplastic polymer substrates are laminated to the template film so that a plurality of microprotrusions project from each of the thermoplastic polymer substrates into the holes and bond the two polymer substrates together through the tips of the microprotrusions. The thermoplastic polymer substrates are then delaminated from the template film while maintaining the thermoplastic polymer substrates in a sufficiently softened state to stretch the microprotrusions into microfibers prior to debonding of the thermoplastic polymer substrates from each other. Microfibers formed via this method typically have a tapered profile.
Another method which may be used to produce a unitary polymer film includes laminating a carrier film to a non-porous thermoplastic polymer film. The carrier film is then pulled away from the polymer film while maintaining the thermoplastic polymer in a sufficiently softened state to allow a portion of the polymer film to be pulled and stretched into a plurality of high aspect ratio microfibers (e.g., microfibers that resemble an extremely thin xe2x80x9cangel hair pastaxe2x80x9d). The high aspect ratio microfibers extend from and are integral with the thermoplastic polymer surface. Napped polymer surfaces of this type are characterized by substantially all of the microfibers (i) having a tip end and (ii) being integrally connected to the underlying polymer surface at their base. As used herein, xe2x80x9ctip endxe2x80x9d means that portion of the microfiber which is furthest from the base along a path that starts at the base and runs length-wise along the fiber. The microfibers generated by this method typically have an aspect ratio of at least about 10 and preferably at least about 20, however, microfibers having an aspect ratio greater than 100 can generated by this method.